> My thanks to everyone who gave their reasons.
>
> My preference is for birth certificates because they give the parents
> names, which makes it easier to find the marriage and also the correct
> family in the censuses together with other children; or only the
> mother's if the child is illegitimate which saves a fruitless search
> for a marriage but which can still find the child's grandparents.
>
> Marriages come in second for confirmation of the right couple and for
> the father's names (providing the father isn't invented to hide
> illegitimacy).
>
> Death certs trail in a distant third and are often bought for children
> to confirm the father if I can't find a birth registration or for a
> young adult to satisfy my curiosity about the cause of death.

My preference is for marriage certificates.
These can have some details about 4 people, i.e.bride, groom and both
fathers.
As well as that there may be clues to other family members from the
witnesses' names or the fact that there was a previous marriage for one or
both of the couple.
The addressses and/or names and occupatons of the fathers can help
to find the couple on the census with their family, or to help find the
fathers
on the census.

A birth certificate usually only has the names of 3 people, and only one
address.... though I do have one where the mother's father registered the
birth as it was in his house.

I have bought a death certificate for a child rather than a birth
certificate
to make sure it was that child who had died and not one of the same name.